Method of producing the alkali metals



51511.21 1 936, I v M, Q N 2,028,390

METHOD OF PRODUCING THE ALKALI METALS Filed Sept. 29, 1955 MPOR fiesssum: Cum .6

MM. fiezssuke no 300 4- '6' 7n 3 [on low 2662655 wz INVENTOR.

Patented Jan. 21, 1936 UNITED STATES METHOD OF PRODUCING 'rnn ALKALIMETALS Miles G. Hanson, Flint, Mich.

Application September 29, 1933, Serial No..69l,491

4 Claims.

This invention relates to a process for economically producing metals ofthe alkali group, particularly lithium. It also describes a modificationof the process by which mixtures of the alkali metals with magnesium orcalcium may readily be made. Lithium and the other alkali metals arecustomarily produced by electrolysis of the fused salts. The process isexpensive to carry out, particularly in the case of lithium because ofits light weight and low melting point, 186 degrees centigrade whichmake it very difficult to separate from the heated fused salts of higherspecific gravity.- Its extreme avidity for oxygen and nitrogen also makerecovery by this method hard to accomplish without large losses of themetal.

This process is-based on the well-known fact that the alkali metals maybe displaced from their oxygen and chlorine compounds by the reducingaction of. magnesium, aluminum or calcium. Potassium and sodiumcompounds are reduced by the action of iron, although this action doesnot 'take place with lithium, caesium or rubidium compounds. Thesereducing reactions take place i with increasing violence as the atomicweight of the element is less, the reaction with lithium compounds beingpractically explosive. Small amounts of the alkali metals have beenformed by conducting the reaction in a sealed container to exclude theair, the reaction being initiated by heating the container and the metalvolatilized by the heat of the reaction condensed in a cooled portion ofthe reaction chamber. The state of the art is well summarized in MellorsTreatise on Inorganic and Theoretical Chemistry, vol. 2, pg. 450. Theproduction of any considerable amount of metal by these reductionprocesses was found by me to be impossible because the reaction resultsin the liberation of gases which rapidly vbuild up the pressure in thesealed container so that the vapor pressure of the metal is exceeded andthe metal is not volatilized but remains mixed in finely divided formwith the other products of thereaction. In the case of. lithium thereaction proceeds so violently that the reduction of more than a fewounces of the material is not possible and practically no lithium metalis distilled from the reacting mass. In order to determine whether thealkali metals, particularlylithium, could be made economically by thisprocess, a study of the factors affecting the process was made. It wasrealized that inorder to remove the metal set freeby the reducing actionthe metal must be volatilized and therefore the pressure at which thereaction takes place must not exceed the vapor pressure of the freedmetal at the temperature of the reacting mass. Figure I shows the vaporpressures of certain of the alkali metals as they are dependent ontemperature. The vapor pressure of magnesium is also included in thisfigure. Experiment showed that the reaction between any of the alkalimetal-oxygen compounds and magnesium resulted in an immediate pressurerise within a sealed container and that the pressure could only be keptlow enough to insure complete volatilization of the freed al-- kalimetal by continuously removing the gas set free by the reaction. In thecase of lithium, for instance, the pressure must be kept below 6 mm ofmercury if the temperature of the reactin mass is 900 degreescentigrade.

I have found these reducing reactions to proceed'at a very rapid rate,this being particularly so in the case of lithium compounds reduced withcalcium or magnesium. If any quantity of the metal is to be prepared thepressure cannot be kept low enough to insure volatilization of even asmall portion of ,the alkali metal that is freed from its compound, eventhough very high ca-- pacity vacuum'pumps be used. It has been proposedto slow down these reactions by admixing inert materials to the reactingmass, such as calcined magnesium oxide, but the addition of even aslarge an amount as 50% by weight of the total charge did not reduce thereaction rate enough to be of value in maintaining a. low pressure inthe retort. The effect of grain size on the rate of re action wasinvestigated and it was found that the grain sizeiis an importantfactor. Thus, the reaction between lithium oxide and magnesium, bothground to pass a mesh per inch screen, is extremely rapid, the use ofparticles of magnesium just able to pass a 25 mesh per inch screenresults in a slow reaction taking several minutes for completion. Duringthe slow progress of the reaction with large grain materials aconsidera-- ble portion of the magnesium is volatilized and thereduction of the oxide is not complete. The condensed metal is found mma mixture of lithium and magnesium. Thus the control of the grain sizeof the mixture affords control of the condensed metal composition aswell as of the reaction rate.

To secure the pure alkali metals it was found that rapid, completereduction of the'alkali metal compound was essential, and that thereaction take place while the pressure within the reaction chamber iskept low. A reaction that takes place under such conditions of pressureand temperature as to fail to volatilize the metal formed will notresult in eflicient recovery of the metal. .55

- paratus shown in Figure 2 was developed. The

vacuum retort above I50'degrees centigrade over the lower two thirds ofits length and is provided with a water cooled cover 2, upon which thevolatilized metal is condensed. The pressure is reduced by a vacuum pump1, of large capacity connected directly to the retort, and the pressureis continuously recorded by a pressure gauge, 3. is provided with a sidetube opening into it, 4, and provided with a sliding member enteringthrough a vacuum tight stufling box. This rod is for the purpose ofinjecting the prepared reacting materials into the vacuum retort as thereaction progresses. The material, consisting of a mixture of alkalimetal compound and the reducing metal, is compressed into pellets, 5,which are placed in the side tube. In operation the retort is heated to750-800 degrees centigrade, the side tube is filled with pellets of thereacting materials and the pressure in the retort is reduced to below 1mm. of mercury. The reaction is started by forcing the rod, 6, inward topropel a pellet into the retort where reaction takes place. The reactionresults in a rise of pressure as indicated by the gauge. The pressure isprevented from rising too far by the continuous operation of the pump,and as each pellet completes its reaction another is pushed into theretort. The size of the pellet is determined asthat which does not causeexcess pressure rise with the pump and size of reaction chamber used. Asan example a mixture of lithium oxide and magnesium, both powdered topass a 100 mesh per inch screen, can be charged into a retort of 2liters capacity and a pump able to remove 5 liters of gas per minute isable to maintain the required pressure if the pellet does not exceed, 10grams in weight.

After all the material has reacted the retort is cooled to roomtemperature and the top removed in an inert gas atmosphere. Thecondensed metal may thus be removed without danger of fire.

During the course of these experiments we have found that either thecarbonate, hydroxide,

or oxide of the alkali metal may be used. We prefer the oxide where afinal pure product is desired. The carbonate tends to produce anappreciable amount of carbon in the deposit, and the hydroxide has thedisadvantage of producing a more explosive reaction, charge that may beemployed. While we do not limit ourselves thereto, we prefer tomanufacture lithiumfrom the oxide as obtained by the action of hydrogenon molten lithium carbonate. It is preferable to heat the carbonate to750-800 degrees centigrade in hydrogen for a sufflcient time to reducethe carbonate completely to oxide. This process has the additionaladvantage that the amount of lithium carried into the retort by eachpound of lithium oxide is much greater than in the case of the carbonateor hydroxide, which increased efliciency is claimed as part of thepresent invention. The carbonate is the most readily available of thecompounds of lithium and by the process in accordance with the presentinvention it is made available for a reduction for which it has notheretofore been used commercially.

While it. is preferable to use magnesium as a reducing agent in thepreparation of lithium, caseium or rubidium, either calcium or aluminummay be employed instead of magnesium I, is maintained at a temperature;

The retort thus lessening the v without departing from the scope of theinvention.

I have found that the composition of the condensed metal recovered canbe predetermined and varied from substantially pure lithium, to yieldscontaining as high as 90% of magnesium by varying the proportion and/orgrain size of the alkali metal compound and the magnesium employed inthe reaction.

For certain purposes the use of lithium mixed with magnesium ispreferable to the pure metal as difliculty and danger in handling areeliminated, this being particularly the case in metallurgicalapplications where lithium is added to molten metals at hightemperatures.

The making of an alloy of lithium with magnesium or calcium by mixingthemolten metals is difficult and dangerous. By the present invention thisstep is eliminated, as a mixture of the alkali metals with magnesium orcalcium can be condensed as the final step in my process, the mixturecharged into the retort being properly proportioned.

As an example, one and one half parts of magnesium to one part oflithium oxide by weight will produce metal containing about 20%magnesium. The mixture may be varied to suit the composition desired, orpellets of pure magnesium may be alternated with pellets of the mixture,though in the latter case the product is not as uniform, but tends to bedeposited in layers on the cooled portion of the retort.

To render the handling of the alkali metal less dangerous a final pelletof magnesium may be charged into the retort, this coats the alkali metaldeposit with a thin coating of magnesium and protects the deposit fromair or gases that would react with the alkali metal as the retort is inprocess of being unloaded.

Having now described the invention, what is claimed is:

1. The process of producing lithium meta l by the reduction of lithiumoxide by metallic magnesium or aluminum, the reduction being carried outat a pressure below 6 mm. of mercury, and within a temperature range of750-900 degrees centigrade, and the metal being recovered bycondensation of the vapor on a cooled portion of the apparatus.

2. The process of producing lithium metal consisting in heating amixture of lithium oxide and not more than an equal amount by weight ofmagnesium metal to a temperature or 750 to 900 degrees centigrade, thereaction being carried out at apressure of not more than 6 mm. ofmercury.

3. A process for producing lithium metal comprising the following steps;production of lithium oxide from lithium carbonate by heating thecarbonate in the presence of hydrogen to 750-900 degrees centigrade,reduction of this oxide product by ,metallic magnesium at a temperatureof 750-900 degrees centigrade under a pressure of not more than 6 mm. ofmercury, the vaporized lithium being condensed on a cooled portion ofthe container.

4. The process of producing an intimate mixture of finely dividedmetallic lithium and magnesium, consisting in heating a mixture oflithium oxide and magnesium to a temperature of 750-900 lithium oxide byweight.

MILES G. HANSON.

